The present invention relates to exposure masks used in fabricating microfine patterns for semiconductor devices, superconducting devices, magnetic devices, optoelectronic devices and various other devices. The invention also relates to methods of fabricating such exposure masks and methods of correcting them, and methods of using them in fabricating the above-referred-to devices.
A reduction projection exposure method has been a commonly used method for fabricating microfine patterns in such devices as LSI circuits. Among the proposed techniques that employ the reduction exposure projection method in drastically improving the resolution is a method using a phase-shifting mask, which introduces a phase shift between exposure lights that have passed through adjacent transparent areas on the mask.
This method, in a case where a pattern consists of slender transparent and opaque areas occurring alternately, provides a transparent material (referred to as a phase shifter) on every other transparent area to introduce a phase shift of about 180 degrees between the lights that have passed through the adjacent transparent areas on the mask. The phase-shifting mask can be manufactured by providing a phase shifter on a particular transparent area on a conventionally used chrome mask. This is discussed, for example, in IEEE Trans. on Electron Devices, ED29, No. 12 (1982), pp. 1828-1836.
However, it is difficult to apply the above-mentioned method to all patterns. For example, when the phase-shifting mask is applied to a pattern of U-shape lying on its side, the arrangement of the phase shifter pattern will be as shown in FIG. 12, in which a pattern area 8 with a phase shifter and a pattern area 9 without it are directly in contact with each other. Because lights passing through these areas cancel each other at the boundary portion of the areas, a desired pattern cannot be transferred.
With the above-mentioned technique, since the phase shift is changed stepwise, the resist pattern is constricted at a boundary area where the phase changes, due to a sharp change of light intensity, giving rise to lack of uniformity and a possibility of pattern break. Also, defects in the phase shifter will transfer the defects as irregular resist patterns or defects.
A limitation may arise as to patterns to which the method of providing a 180 degree phase shifter can be applied. As a means to alleviate such a limitation, there is a method which changes the phase shift stepwise from 0 degree to 180 degrees, as mentioned in the Japanese Laid-Open No. 34854/1990. This method utilizes the fact that the interference effect of the exposure light decreases as the phase shift decreases from 180 degrees, so that an opaque region likely would not be formed.
As a method of fabricating such a phase-shift mask, the Japanese Patent Laid-Open No. 34854/1990 cites a method which performs sputtering a number of times to change stepwise the phase shift of the phase shifter material such as a siliceous film, a method which etches the phase shifter film and mask substrate stepwise, and a method which repeats the processes of resist coating, resist pattern forming and hardening. The former two methods, however, since they must perform the sputtering and etching a number of times, make the mask forming process complex. With the third method, as shown in FIG. 8 of the Japanese Patent Laid-Open No. 34854/1990, an area for giving a 180 degree phase shift is formed by performing sputtering and coating a number of times. Not only does this render the mask manufacturing process complicated but accumulates film thickness variations due to sputtering and coating, so that it may become difficult to precisely control the phase shift at exactly 180 degrees.
With the above-mentioned techniques, the mask structure and the mask making process become complicated.
As a further proposed technique to alleviate the limitation of pattern variations to which the phase-shifting mask can be applied, there is a method which provides an area of, say, 90 degree phase shift in a boundary portion between the 0 degree phase shift area and the 180 degree phase shift area. The arrangement of the mask patterns according to this method is shown in FIG. 13. A first transparent pattern area 10 is a transparent area with no phase shifter, a third transparent area 12 is an area with a phase shifter which produces a phase shift of 180 degrees, and a second transparent area 11 is an area with a phase shifter which produces a phase shift of 90 degrees. Since the phase shift between the exposure lights that have passed through the adjacent transparent areas is 90 degrees, the light shielding effect due to interference of light at the boundary portion is weakened. Therefore, the desired U-shaped pattern can be transferred. This is described in Japanese Patent Laid-Open No. 34854/1990.
To form a phase shifter pattern with two phase shifts--180 and 90 degrees--as shown in FIG. 13, a conventional method uses an electron beam resist material as the phase shifter material and regulates the electron beam exposure dosage to control the phase shift, i.e., the thickness of the phase shifter film. Information on this subject was made public in the 35th International Symposium on Electron, Ion and Photon Beams, (1991), L1.
In addition, in fabricating devices such as DRAMs using the above-mentioned phase-shifting mask, defects in the mask pattern significantly lowers productivity. Therefore, it is essential to develop a technique to manufacture masks without defects or a technique for correcting the defects perfectly.
In the case of a phase-shifting mask, any defects in the phase shifter will not only make it impossible to produce a normal phase shift effect but will transfer this part as a defect or an irregular pattern. Hence, a technique to correct the phase shifter defects is also needed.
A proposed method of correcting defects in the phase shifter pattern of the phase-shifting mask is to make the mask structure easy to correct so that the defects can be corrected by using a selective etching technique, using, for example, a focused ion beam. An illustrative structure may have a second phase shifter layer for defect repairing between the phase shifter and the mask substrate. This structure is introduced in the 51st Lecture Meeting of Applied Physics Society, 2-volume collection of lecture texts, page 493, 2-p-ZG-10.